This invention concerns improvements in or relating to optical display apparatus.
In some circumstances there is a requirement for displaying two sets of visual information simultaneously in superimposed manner to an observer. For example, it is sometimes required in a head-down display to superimpose a map display on e.g. a radar display on a cathode ray tube (CRT) phosphor, or in a head-up display to superimpose a stand-by display of a graticule or the like on a main display on a CRT phosphor. Conventionally such superimposition is effected by some form of beam-splitter arrangement whereby light from one display, e.g. the CRT, is transmitted through the beam-splitter while light from the other display, e.g. the map or stand-by, is reflected from the beam-splitter. These beam-splitter arrangements generally tend to entail space, weight and cost penalties. There are other more simple options but these also tend to have their disadvantages. For example, a head-down display may have a CRT with a fibre optics faceplate and a transparency, e.g. showing a map, in contact with it. This means a more expensive CRT and puts constraints on the display itself since it alone illuminates the transparency. In a head-up display it has been proposed to focus light from a stand-by display source on to the CRT phosphor of the main display but this can have brightness problems.